Method and circuit arrangement for operating a high-pressure gas discharge lamp

ABSTRACT

A method and a circuit arrangement for the operation of a high-pressure gas discharge lamp (HID [high intensity discharge] lamp or UHP [ultra high performance] lamp) is described, which lamp is particularly suitable for illuminating projection displays with sequential color rendering (for example LCOS or SCR-DMD systems) with a pulsatory lamp current. Artefacts in the color rendering are avoided through the generation of at least one compensation pulse of a given amplitude and a given timing and through superimposition thereof on the lamp current.

The invention relates to a method and to a circuit arrangement foroperating a high-pressure gas discharge lamp (HID [high intensitydischarge] lamp or UHP [ultra high performance] lamp) such that thelatter is designed in particular for illuminating projection displayssuch as, for example, LCOS (liquid crystal on semiconductor) or SCR-DMD(sequential color recapture-digital micro mirror) color displays. Theinvention also relates to a projection system with a projection display,a high-pressure gas discharge lamp, and such a circuit arrangement.

A method and a circuit arrangement for operating a high-pressure gasdischarge lamp is disclosed in U.S. Pat. No. 5,608,294. According tothis publication, the lamp is operated with an alternating current, bymeans of which a fast erosion of the electrodes can be prevented and theefficacy of the lamp can be enhanced. Such an alternating current,however, also increases the risk of unstable arc discharges, which maylead to a flickering of the generated luminous flux. This finds itsorigin essentially in the fact that the arc discharge is dependent onthe temperature and the condition of the surface of the electrodes andthat in addition the time gradients of the electrode temperature aredifferent for the phases in which the electrode acts as an anode and asa cathode. This again has the result that the electrode temperaturechanges considerably during one cycle of the lamp current. To eliminatethis problem to a substantial degree, a current pulse is generated atthe end of each half cycle of the lamp current, i.e. before a polaritychange, which pulse has the same polarity and is superimposed on thelamp current, so that the total current is increased and the electrodetemperature rises. The stability of the arc discharge can beconsiderably improved thereby.

This current change, however, also has the result that the lamp is nowoperated with an alternating lamp current which comprises more or lessstrongly accentuated pulsatory components, which in their turn cause acorrespondingly pulsatorily increased luminous flux. This, however, maylead to artefacts in particular if such a lamp is used for illuminatinga projection display with sequential color rendering.

This relates, for example, to LCOS displays, in which the three primarycolors run sequentially over the display in the form of color bars (cf.Shimizu: “Scrolling Color LCOS for HDTV Rear Projection” in SID 01Digest of Technical Papers, vol. XXXII, pp. 1072 to 1075, 2001).Whenever the luminous flux rises owing to a current pulse, thebrightness of the color bars rises correspondingly. As a result, thecolors are always represented with a higher brightness in certainregions of the display than in other regions of the display, independence on the instantaneous positions of the color bars. To achievea good picture quality, however, the brightness of the three colorsshould be equal in all picture regions, in particular if the alternatinglamp current is synchronized with the image repetition frequency foravoiding interference or similar effects.

The SCR-DMD projection displays are also affected by the above artefacts(cf. Dewald, Penn, Davis: “Sequential Color Recapture and DynamicFiltering: A Method of Scrolling Color” in SID 01 Digest of TechnicalPapers, vol. XXXII, pp. 1076 to 1079, 2001).

It is accordingly an object of the invention to provide a method and acircuit arrangement for the operation of a high-pressure gas dischargelamp with which a particularly homogeneous luminous flux can begenerated, also when the luminous flux is averaged over a comparativelyshort period of time.

In particular, a method and a circuit arrangement for operating ahigh-pressure gas discharge lamp with a pulsatory lamp current is to beprovided by means of which in particular projection displays can beilluminated such that a substantially natural color impression iscreated.

Furthermore, a method and a circuit arrangement for operating ahigh-pressure gas discharge lamp with a pulsatory lamp current is to beprovided by means of which in particular projection displays can beilluminated without substantial visible artefacts or other visuallyobservable interferences.

Finally, a method and a circuit arrangement are to be provided by meansof which a high-pressure gas discharge lamp can be operated such thatthereby not only an artefact-free color rendering is achieved with aprojection display having sequential color rendering, but also aflicker-free luminous flux with a stable arc discharge can be generated.

The object is achieved according to claim 1 by means of a method ofoperating a high-pressure gas discharge lamp wherein the lamp is fedwith a lamp current on which are superimposed at least first currentpulses and at least one second current pulse associated with each firstcurrent pulse, wherein said first and second current pulses haveamplitudes in mutually opposed directions and a definable timedifference between them, and wherein the number and/or the level of theamplitude and/or the time length of the second current pulses is/areadjusted such that the changes in the luminous flux caused by the firstcurrent pulse and by the at least one respective associated secondcurrent pulse compensate each other at least substantially.

The object is further achieved by means of a circuit arrangement asclaimed in claim 6.

The fact that a luminous flux raised by, for example, a first currentpulse is compensated by one or several second current pulses, which leadto a corresponding reduction in the luminous flux because of theiropposed directions and their superimposition on the lamp current,renders it possible to generate a very homogeneous luminous flux,averaged over a (short) period of time, in particular if the timedistance between the first and second current pulses is comparativelysmall.

A compensation is to be regarded as being achieved when—depending on theapplication of the lamp—the artefacts or other interferences mentionedabove are no longer perceivable.

The dependent claims relate to advantageous further embodiments of theinvention.

The distance in time between the first and the second current pulses ispreferably chosen in accordance with claims 2 and 7 in the case of alamp application for illuminating a projection display with sequentialcolor rendering. A particular advantage of these solutions is thatartefacts can be reliably avoided in a comparatively simple mannerthereby and for substantially any cycle durations of the primary colors(subframe frequencies) of a projection display, without appreciablelimitations having to be accepted as regards a current waveformoptimized for the lamp operation in question.

The embodiments of claims 3 and 4 essentially have the advantage that ahigh-pressure gas discharge lamp is operated thereby on the one handwith a lamp current which is optimized, for example, as regards ahomogeneous electrode erosion (alternating lamp current) and aflicker-free operation (additional current pulses), as described, forexample, in U.S. Pat. No. 5,608,294, but which on the other hand canalso be used in the lamp application for illuminating displays withsequential color rendering without artefacts being caused by thedifferent pulse components.

Claim 5 renders possible a particularly simple embodiment of the method.

The circuit arrangement of claim 8 renders it possible to implement themethod according to the invention in a comparatively simple andinexpensive manner.

Further details, features, and advantages of the invention will becomeapparent from the ensuing description of preferred embodiments, which isgiven with reference to the drawing, in which:

FIG. 1 shows the time gradient of the color activation and of a luminousflux in a line of a display;

FIG. 2 shows a first basic function for compensating an increasedluminous flux;

FIG. 3 shows a second basic function for compensating an increasedluminous flux;

FIG. 4 shows a third basic function for compensating an increasedluminous flux;

FIG. 5 is a time diagram of an absolute and a relative luminous flux inaccordance with the first basic function;

FIG. 6 shows a time gradient of an alternating lamp current withcompensation pulses for the case shown in FIG. 5;

FIG. 7 shows a time gradient of a relative luminous flux with acombination of three of the first basic functions;

FIG. 8 shows a time gradient of an alternating lamp current withcompensation pulses for the case shown in FIG. 7;

FIG. 9 shows a time gradient of a relative luminous flux with acombination of two of the second basic functions;

FIG. 10 shows a time gradient of an alternating lamp current withcompensation pulses for the case shown in FIG. 9;

FIG. 11 shows a frequency spectrum of the illumination of a display forthe alternating lamp current shown in FIG. 10; and

FIG. 12 shows a circuit arrangement for generating an alternating lampcurrent.

To clarify the general problem, the following observations are to bemade first.

When a color display of the kind mentioned above is illuminated with alamp whose supply current is superimposed with current pulses which leadto a corresponding pulsatory increase in the generated luminous flux(denoted first current pulses hereinafter), an uneven intensitydistribution of the individual colors over the display may arise.

This is true in particular in the case of an alternating lamp current ifthis current is synchronized with the repetition rate of the primarycolors (color bars), i.e. the subframe frequency, so as to avoidfluctuations in the picture, because this synchronity is then also givenfor the first pulses acting on the lamp current.

A luminous flux intensified in a pulsatory manner thus always hits thedisplay when the three color bars have the same respective positions onthe display, i.e., for example, when the blue color bar lies in theupper third, the green color bar in the central third, and the red colorbar in the lower third of the display. This means that the blue colorswill always have a higher brightness in the upper third, the greencolors in the central third, and the red colors in the lower third ofthe display than they have in the respective other regions of thedisplay.

Artefacts arising in this manner or other visually perceivableinterferences are to be prevented by the invention, and an at leastsubstantially natural color rendering is to be achieved.

A basic idea of the invention is that the color brightness of one colorbar increased by a first current pulse of the kind mentioned above iscompensated in the relevant regions of the display in that thisbrightness is correspondingly reduced when the color bars have reachedthe same display regions again in one (or several) subsequent subframecycle or cycles. This is achieved in that a current pulse issuperimposed on the lamp current at the relevant moment or moments,which pulse (denoted the second current pulse hereinafter) reduces thelamp current and thus also the generated luminous flux correspondingly.

Owing to the high subframe frequency, which is at least three times therepetition frequency of the image (video frequency), the alternatingdifferent brightnesses of one color in one and the same region of thedisplay are not perceivable to the human eye, but are averaged to thebrightness level obtaining in those phases of the lamp current in whichsaid pulses do not occur, i.e. to the brightness level of the respectivesame color in other regions of the display.

FIG. 1 shows the simplest case of this compensation for one line of adisplay. The transmissivity of the individual color segments red (I),green (II), and blue (III) is plotted on the vertical axis, whichsegments transmit red, green, and blue light, respectively, one afterthe other in time. Furthermore, this Figure shows the time gradient ofthe luminous flux (IV, absolute luminous flux) with superimposed pulses.A first pulse (IVa) increasing the luminous flux has the result that thered color segment activated at this very moment lights up particularlystrongly. This increased color brightness is compensated by a secondpulse (IVb) which leads to a correspondingly lower luminous flux of thelamp and which is generated in the next phase in which the red colorsegment is activated. Averaged over time, accordingly, a homogeneousillumination of the display with the various colors is achieved withoutartefacts or other visually perceived interferences occurring.

In dimensioning a circuit arrangement for generating a suitable lampcurrent and for operating a discharge lamp, it is necessary to take intoaccount the following requirements and parameters for optimizing thepicture quality: the length in time of the second (current) pulsesgenerated for compensation should be equal to the length of the first(current) pulses. The frequency, and thus the time shift of the secondpulses, should be activated with the same colors in the same locationsof the display each time, in accordance with the subframe frequency orthe subframe cycle (or a multiple thereof).

It should also be observed that a second current pulse, i.e. theamplitude thereof, cannot exceed the level of the lamp current duringthe pulse-free phases. If the lamp current during the first currentpulse is higher than twice the lamp current in the pulse-free phasesunder certain operational conditions, it is necessary to generateseveral second current pulses each with a sufficient amplitude and withthe distance in time mentioned above (assuming that the lamp currentcannot be limited accordingly during the first pulse).

It is furthermore required in the case in which the lamp is operatedwith a lamp current of alternating polarity, for avoiding a fast andirregular erosion of the electrodes, or for other reasons, that thearrangement in time of the current pulses takes place such that a firstcurrent pulse is generated each time before a change in polarity of thelamp current, which pulse has the same polarity as the instantaneouslamp current and thus increases the latter. Instabilities in the arcdischarge and an accompanying flickering can be avoided thereby.

It should also be observed that no low-frequency components becomevisible on the display, superimposed on the pulse frequencies andleading to interferences. Finally, the limit frequency of the lamp andof the entire projection system including the display should also betaken into account in determining the level of the pulse frequencies.

FIGS. 2 to 4 show three different possibilities of the compensation(basic functions) of a luminous flux increased by a first pulse. Incontrast to the representation in FIG. 1, the vertical axis now showsonly the change in luminous flux (relative luminous flux) caused by thepulses (i.e. the difference between the brightnesses generated by thepulses and by the non-pulsed lamp current). The horizontal axis isstandardized each time to the number of full passages through all colorbars on the display, i.e. the subframe frequency. The basic functionsshown in FIGS. 2 to 4 may also be combined with one another.

In detail, a first pulse is compensated in FIG. 2 by a second pulse ofthe same amplitude and length in the next subframe in the same location.As is shown in FIG. 3, a first pulse is compensated by two second pulsesof the same length and half the amplitude in the two subsequentsubframes. In FIG. 4, finally, a first pulse is compensated by threesecond pulses of the same length and one third of the amplitude of thefirst pulse in the three subsequent subframes. The amplitudes of thesecond pulses always have a direction opposed to that of the amplitudeof the first pulse.

It is also possible to use more than three second pulses forcompensation. This, however, also increases the proportion oflow-frequency components in the light radiation, so that the risk ofvisible artefacts arising is also increased thereby.

Furthermore, the individual pulses may be generated substantially at anydesired locations within a subframe. The determining factor isexclusively the distance in time of the pulses with respect to oneanother, which should correspond as exactly as possible to the timeduration of one subframe (or a multiple thereof). It is thus alsoconceivable to carry out a compensation through generation of a secondpulse in the next subframe but one.

FIG. 5 once more shows the time gradients of the absolute (I) and therelative (II) luminous flux for the first basic function shown in FIGS.1 and 2, and FIG. 6 shows the gradient in time of a correspondingalternating lamp current for realizing this compensation. Given acertain subframe frequency, the cycle duration of the alternating lampcurrent and its phase angle is preferably laid down and synchronized forsafeguarding the stability of the arc discharge such that a first pulseis always generated with the same polarity as the instantaneous lampcurrent before a change in polarity takes place.

If the frequency of the alternating lamp current is to be increasedrelative to the subframe frequency, additional first pulses are to beinserted, by means of which the stability of the arc discharge issafeguarded, as was mentioned above.

It should be observed during this, however, that the lamp currentresulting therefrom may comprise DC components under certaincircumstances. For example, if two pulse sequences of FIG. 2 arecombined, two first pulses and two second pulses will always follow oneanother. Since it is advantageous for lamp operation to invert thecurrent direction after each first pulse, this would lead to a DCcomponent in the lamp current. The combination of three pulse sequencesof FIG. 2, or the combination of two pulse sequences of FIG. 3 makes itpossible to avoid a DC component.

FIG. 7 shows the relative luminous flux in a combination of three basicfunctions of the kind shown in FIG. 2, involving a phase shift ofapproximately ⅔ subframe each, such that within one subframe a first andtwo second, and in the next subframe two first and one second pulse arepresent. FIG. 8 shows the corresponding gradient of the alternating lampcurrent. Given a subframe frequency of 180 Hz, a lamp frequency of 135Hz is obtained.

As was noted above, it may occur that a first pulse cannot becompensated by only one second pulse. In this case, at least one of the(second and third) basic functions as shown in FIG. 3 or 4 is to beused.

If only one such basic function is used, however, a comparatively lowlamp frequency will be the result. For example, only one first pulsearises within three subframes in the compensation shown in FIG. 3, sothat a subframe frequency of 180 Hz will lead to a lamp frequency ofonly 30 Hz. A linear combination of the basic functions is to bepreferred for this reason.

FIG. 9 shows the relative luminous flux in a combination of two (second)basic functions of the kind shown in FIG. 3, which have a phase shift of1.5 subframe with respect to one another. A time gradient of the lampcurrent as shown in FIG. 10 is the result of this.

FIG. 11 shows the amplitudes of the various frequency components thatoccur when a display is illuminated by a lamp having the lamp currentshown in FIG. 10. In FIG. 11, circular dots indicate frequencycomponents caused by the modulation of the DC component of the displayillumination when the color bars are traversed, and triangular dotsindicate the frequency components caused by the first and second pulses.Since the luminous flux cycle in this case covers three subframes, andthe subframe frequency is assumed to be 180 Hz, the lowest frequencycomponent of the pulses lies at 60 Hz.

FIG. 12 finally is a block diagram of a circuit arrangement forgenerating the lamp currents described above. The circuit arrangementessentially comprises a converter 10 known per se (buck converter) forgenerating a direct current from the supply voltage obtained from a DCvoltage source 11, a control device 20 for controlling the converter 10such that the direct current will have a gradient as described above,and a commutator 30 for converting the direct current of the converter10 into a suitable alternating lamp current, as well as possibly forgenerating an ignition voltage for a connected lamp 31.

In detail, the converter 10 comprises a series-connected inductance 102and at the output thereof a parallel capacitor 103. The inductance 102is connected to a pole of the DC voltage source 11 in a first switchingposition of a pole changing switch 101 (usually implemented as atransistor or a diode). In a second switch position, the inductance 102is connected in parallel to the capacitor 103. A current measuringdevice 104 is further provided, which generates a current signal whichrepresents the level of the current flowing through the inductance 102.

The control device 20 substantially comprises a microcontroller 201 anda switching unit 202.

A voltage signal obtained from the output of the converter 10 is appliedto an input of the microcontroller 201. The microcontroller 201generates a reference signal (required value for the lamp current) at afirst output, which signal is supplied to the switching unit 202, and acurrent direction signal at a second output, which current directionsignal is applied to the commutator 30 and by means of which thecommutation of the lamp current is achieved in a synchronized manner.

The switching unit 202 comprises a first logic gate 2021 to whose firstinput the current signal is applied and to whose second input thereference signal generated by the microcontroller 201 is applied, and asecond logic gate 2022, which also receives the current signal. Theswitching unit 202 further comprises a switching element 2023 with a setinput which is connected to the output of the second logic gate 2022,and with a reset input connected to the output of the first logic gate2021. An output Q of the switching element 2023, finally, is connectedto the pole changing switch 101, switching over the latter between itsswitching positions.

The switching device operates substantially as described below, where itis assumed that the process steps relating to the ignition and run-up ofthe lamp are known in the art and need not be explained in detail here.

At the start of a switching cycle of the converter 10, the pole changingswitch 101 is first in the first (upper) switching position in which itconnects the positive pole of the DC voltage source 11 to the inductance102. The current thus flows through the inductance 102 and increasesuntil its level, detected by means of the current signal, exceeds thereference signal (required value for the current) applied to the secondinput of the first logic gate 2021. When this is the case, the firstlogic gate 2021 generates a signal at the reset input of the switchingelement 2023, so that the latter switches over the pole changing switch102 into the second (lower) switching position shown in FIG. 12. Theinductance 102 is separated from the DC voltage source 11 thereby, andat the same time the capacitor 103 is connected in parallel, so that adecaying current now flows in the circuit thus formed. Once this currenthas reached zero value, the second logic gate 2022 generates a signal atthe set input of the switching element 2023, so that the latter switchesover the switch 101 into the first switching position, and the processstarts anew.

The switching frequency of the pole changing switch is essentiallydefined by the dimensioning of the inductance 102 and generally liesbetween approximately 20 kHz and a few hundreds of kHz. The capacitor103 is dimensioned such that the output voltage applied to the converter10 remains substantially constant, so that also the current flowingthrough the commutator 30 and the lamp 31 remains substantially constantand in the ideal case is half the reference value given by themicrocontroller 201. Conversely, the microcontroller 201 must alsogenerate at its first output a current reference signal which is twiceas large as the desired lamp current.

The lamp current gradient is determined on the one hand by its frequencyand on the other hand by the fact that a first current pulse is to begenerated before each polarity change and having the same instantaneouspolarity, as was explained above. In dependence on the first currentpulses, furthermore, the second current pulses should be generated andshould be superimposed on the lamp current in a corresponding manner.The length of the current pulses and the maximum amplitude of the totalcurrent flowing through the lamp during a current pulse are essentiallydefined by the lamp characteristics. All these parameters are stored inthe microcontroller 201 (or in a memory), so that the microcontrollercan generate the current reference signal with the suitable gradient.

The time schedule for synchronization of the current pulses with theimage generation on the display may be variable or constant. Theprocedure for a constant, predetermined time schedule will be describedbelow.

First the microcontroller 201 calculates the required average currentvalue and the current value during the second pulses in a first sequenceof steps from the voltage U_(meas) measured at the output of theconverter 10 and supplied as a voltage signal, the second pulses in thisexample being exactly as long as the first pulses. This first sequenceof steps is preferably repeated at regular intervals.

The microcontroller 201 then first detects whether the measured voltagevalue U_(meas) lies between a minimum and a maximum value. If this isthe case, the microcontroller 201 calculates from this voltage valueU_(meas) and the lamp power P the required average current valueI_(AGV)=P/U_(meas). Then the required current value (I_(comp)) for thesecond pulses is calculated therefrom as well as from the storedamplitude (current value) of the first pulses (I_(pulse)) and the storednumber n_(comp) of second pulses:I _(comp) =I _(AGV) −ΔI _(pulse) /n _(comp), where ΔI _(pulse) =I_(pulse) −I _(AGV)

In a second sequence of steps, the reference signal at the first outputand furthermore the current direction signal at the second output of themicrocontroller 201 is repeatedly generated in accordance with thedesired cycle of the alternating lamp current on the basis of thesethree current values (I_(AGV), I_(pulse)) and I_(comp)) the requiredswitching times being obtained from the memory. It is necessary only toobtain the values of a half cycle each time, because the other halfcycle will always have the same gradient (with reversed polarity). Inthe usual case of a regular distribution in time of the first and secondcurrent pulses, furthermore, only two time values are required, i.e. theinterval between two current pulses t_(const) and the duration t_(pulse)of the current pulses.

More in detail, the reference signal is first set for double the averagecurrent value I_(AGV), so that the lamp current desired for thepulse-free phases is adjusted, as was noted above. After the periodt_(const) has elapsed, the reference signal is set for double thecurrent value I_(comp) required for the second current pulse, so thatthe lamp current will be reduced by the amplitude of the second currentpulse. After the pulse time t_(pulse) has elapsed, this procedure isrepeated n times in the case in which several (n) second current pulsesare to be generated for compensating one of the first current pulses.

If only one second current pulse is to be generated, the referencesignal is also set again for double the average current value I_(AVG) ina next step. After the time t_(const) has elapsed, the reference signalis now set for double the current value I_(pulse) required for the nextfirst current pulse, so that the lamp current is increased by the valueof the first current pulse. After the pulse duration t_(pulse) haselapsed, finally, the current direction signal is generated at thesecond output of the microcontroller 201, so that the commutator 30switches over the current direction of the lamp current and thusinitiates the second half cycle of the alternating lamp current inaccordance with the first and second sequence of steps described above.

The calculations given above were based on the assumption that theluminous flux supplied by the lamp is substantially linearly dependenton the lamp current. This assumption is justified for most high-pressuregas discharge lamps. In other lamps, the current should be calculatedwith an additional correction factor for the second current pulses, asapplicable, so that the degree to which the luminous flux is increasedduring one of the first current pulses is again equal to the degree towhich the luminous flux is reduced during the associated second currentpulse (or the associated total number of second current pulses).

1. A method of operating a high-pressure gas discharge lamp, wherein thelamp is fed with a lamp current on which are superimposed first currentpulses and at least one second current pulse associated with each firstcurrent pulse, wherein said first and second current pulses haveamplitudes in mutually opposed directions and a definable timedifference between them, and wherein the number and/or the level of theamplitude and/or the time length of the second current pulses is/areadjusted such that the changes in the luminous flux caused by the firstcurrent pulse and by the at least one respective associated secondcurrent pulse compensate each other at least substantially.
 2. A methodas claimed in claim 1, in particular for operating a high-pressure gasdischarge lamp which is provided for illuminating a projection displaywith primary colors that are repeatedly generated sequentially with acycle duration, wherein the first and second current pulses have adistance in time from one another which corresponds to one cycle or to amultiple of one cycle of the primary colors.
 3. A method as claimed inclaim 1, wherein the amplitudes of the first current pulses are directedsuch that they generate an increase in the generated luminous flux,which increase is at least substantially compensated in that acorresponding reduction of the generated luminous flux is effected bythe at least one respective associated second current pulse.
 4. A methodas claimed in claim 3, wherein the lamp current is a substantiallysquare-wave alternating current on which the first current pulses aresuperimposed before a polarity change of the lamp current each time. 5.A method as claimed in claim 1, wherein the first and second currentpulses all have substantially the same length in time.
 6. A circuitarrangement for operating a high-pressure gas discharge lamp bygenerating a lamp current, by generating first current pulses andsuperimposing them on said lamp current, and by generating at least onesecond current pulse associated with each first current pulse, whereinsaid first and second current pulses have amplitudes in mutually opposeddirections and a definable time difference between them, and wherein thenumber and/or the level of the amplitude and/or the time length of thesecond current pulses is/are adjusted such that the changes in theluminous flux caused by the first current pulse and by the at least onerespective associated second current pulse compensate each other atleast substantially.
 7. A circuit arrangement as claimed in claim 6, inparticular for operating a high-pressure gas discharge lamp which isprovided for illuminating a projection display with primary colors thatare repeatedly generated sequentially with a cycle duration, wherein thefirst and second current pulses have a distance in time from one anotherwhich corresponds to one cycle or to a multiple of one cycle of theprimary colors.
 8. A circuit arrangement as claimed in claim 6,comprising a converter (10) for generating the lamp current from asupply voltage, comprising a control device (20) with a microcontroller(201) for controlling the converter (10) in dependence on a voltagesignal at the output of the converter (10), and furthermore independence on a current signal which represents the amplitude of acurrent flowing through the converter (10), and furthermore independence on a nominal time course of the lamp current stored in themicrocontroller (201) please, adapt in description, too.
 9. A projectionsystem with a projection display, at least one high-pressure gasdischarge lamp, and a circuit arrangement as claimed in claim 6.